GROWTH OF THE CROP 263 



frequently involve no drastic disturbance of accepted routine. 

 It also has to be remembered that the present trend of agricultural 

 conditions in many countries is such as leaves a narrowing margin 

 of expenditure available for insecticidal treatment, or other more 

 or less costly measures. There is, in consequence, an increasing 

 tendency to study the relations between insect attack and the 

 conditions under which the crops are grown. This in its turn 

 involves the application of ecological methods, and in the present 

 chapter endeavour will be made to show that success, in this 

 direction, is dependent upon adequate knowledge of the biology 

 of the insect in relation to the growth phases of the crop which it 

 attacks. 



In the present instance the application of ecological methods 

 will be discussed under four aspects. (1) The regulation of the 

 growth of the crop ; (2) the production of varieties of plants 

 immune, or highly resistant, to insect attacks ; (3) the bearing of 

 host selection by insects, and the evolution of biological races of 

 the latter ; and (4) the ecological aspects of the locust problem. 

 It is in relation to these four problems that ecology, in the broad 

 sense of the term, enters into closest relations with practical 

 entomology to-day. 



Regulation of the Growth of the Crop 



This procedure aims at regulating the growth of the crop in 

 such a way that it either escapes infestation or is subjected to 

 appreciably lighter attack. In a general way, two methods are 

 applicable. Firstly, the adoption of an early sowing date in 

 order to ensure a given crop is sufficiently advanced in growth to 

 withstand attack at the usual period of the latter. Secondly, the 

 investigation of the nutritional requirements of the crop in order 

 to accelerate its growth at its most vulnerable period, or to 

 promote its growth in such a manner that it becomes less 

 susceptible to infestation. 



Time of Sowing. In the case of certain cereals time of sowing 

 of the crop has a very direct bearing upon their subsequent 

 liability in relation to insect attack. With the frit-fly (Oscinella 

 frit) the earlier the date of sowing in spring the better, providing 



